The pulmonary bronchiolar and alveolar epithelia are involved prominently in a number of important human diseases associated with loss of alveolar integrity, including emphysema and pulmonary fibrosis. In each of these conditions the capacity to generate new alveolar epithelium, and its associated vascular bed, would be of great potential therapeutic value, but such capacity remains beyond the reach of current technology. This application is predicated on the idea that better understanding of distal ainA/ay and alveolar epithelial cell progenitor biology is a crucial part of any effort to move the field of directed distal lung remodeling or repair toward the clinical arena. Toward that end, this application brings together investigative groups with different expertise but overiapplng interests in epithelial progenitor cell biology to advance the understanding of distal lung development, maintenance, and repair. The major objectives are (1) To define the transcriptional program of heretofore uncharacterized distal airway and alveolar progenitors and test the hypothesis that differential expression of adhesion receptors underiies the capacity of epithelial subtypes to self-organize and promote repair. (2) Define the requirement for neuroendocrine cells (PNECS )and alveolar progenitor cells in maintenance and reconstitution of distal airway and alveolar cells following lung injury. (3) Analyze and further develop a novel, single cell in vivo lung organoid assay in kidney capsules in order to optimize the capacity of adult epithelial progenitor cells to generate functional respiratory units de novo. Important tools and approaches developed to achieve these aims include mice with inducible cre activity knocked into lineage-defining genomic loci, flow cytometry-based techniques to isolate and transcriptionally profile mouse and human embryonic and adult epithelial progenitors, and innovative imaging that allows real time capture of stable images of lung and lung organoids over time. We anticipate that by the completion of these studies we should be able to adapt our in vivo assay toward orthotopic transplantation of cellular units capable of lung development. Overall, these studies should provide crucial conceptual and technological infrastructure for the clinical translation of progenitor cell biology to human lung disease.